1. Field of the Invention
This invention is related to a method and apparatus for carrying out the production of graphite electrodes.
Carbon and graphite electrodes are used in many electrochemical processes, including the production of magnesium, chlorine, iodine, phosphorus, steel, and the production of aluminum in Hall cells. The word carbon denotes the amorphous form of carbon, and graphite denotes the multilayered hexagonal crystalline form of carbon.
Carbon electrodes consist of the essentially amorphous carbon from petroleum coke which has been calcined, ground, classified by size, mixed with a binder, and bound in a matrix of amorphous carbon derived from the binder after baking at temperatures of approximately 800.degree. to 900.degree. C. in a baking furnace. Graphite electrodes are produced from the carbon forms by placing them in an Acheson furnace and, in recent years, in a Lengthwise Graphitization (LWG) type furnace and heating them to a temperature between 2500.degree. to 3000.degree. C., which converts the amorphous form of carbon to the crystalline graphite, and vaporizes most of the impurities present in the original carbon, including most metals and sulfur compounds.
Graphite, compared to amorphous carbon, has much higher electrical and thermal conductivity, lower coefficient of thermal expansion (CTE), superior ductility and vastly superior thermal shock resistance at the operating temperatures of the electric arc steel furnace. These physical properties are uniquely valuable in the electric furnace, with its need for high electrical currents, and the need to resist the mechanical and thermal stresses imposed on the electrodes from the falling scrap, fluctuations in metal and electrode level, and generally high thermal stresses. Consequently, graphite is universally used as an electrode in the electric arc melting of steel.
The production of graphite electrodes from the so-called carbon electrodes has traditionally been carried out in the Acheson furnace in which the electrodes are placed in a transverse orientation to the flow of the electrical current, with the current passing alternately through tiers of electrodes and resistor media, the latter being typically metallurgical or petroleum coke. The Acheson process is of such ancient vintage and so well known as not to require any further description. The LWG process, although also very old, is less well known and has been practiced on a commercial scale only in recent years. The LWG process is carried out by arranging the carbon electrodes in a continuous column with an electrical connection at each end of the column. See U.S. Pat. No. 1,029,121 Heroult, June 11, 1912 and U.S. Pat. No. 4,015,068, Vohler, Mar. 29, 1977. The electrodes are arranged so that the electrodes themselves form the dominant path for the heating current, with one or both of the ends of the column subjected to a mechanical or hydrostatic pressure source in order to keep the connection tight under expansion or contraction of the column during the heating cycle. Vohler does not use a packing medium, but discloses a metal shell with a felt linear as insulation.
The LWG process has many advantages over the Acheson process. The energy efficiency is much higher, as the material is heated directly instead of indirectly, and the cycle time for the process is much faster taking typically less than 12 hours as compared to 60 to 120 hours for the Acheson process.